Membrane attack complex (MAC) is an important immune effector of the complement terminal pathway that is composed of complement components C5b, C6, C7, C8, and C9. MAC is the collective come-off of three complement pathways (classical, lectin, or alternative pathway) that penetrates the cell membranes of microbes to form cytotoxic pores. These pores disrupt the phospholipid bilayers of target cell membrane, or form hydrophilic transmembrane channels, leading to cell lysis and death.
Structure and Assembly of MAC
MAC is a multiprotein complex consisting of five terminal complement proteins C5b, C6, C7, C8, and C9. Under electron microscopy, MAC pore measures approximately 10 nm in diameter and is a hollow structure embedded in the surface of pathogen cell membranes. The formation of MAC is initiated by the irreversible proteolytic cleavage of C5 into two components (C5a and C5b) by the C5 convertase. C5b provides a scaffold for C6 binding to form the stable intermediate C5b6, which can spontaneously combine with C7 to form C5b67 (MAC precursor). As a result of this junction, the configuration of C7 is altered, that is beneficial to the exposure the hydrophobic site on C7. Then C7 can insert into the lipid bilayer of cell membranes, where it has a high affinity for C8. Subsequently, C8 binds the membrane-associated C5b67 to form complex C5b678, which has a limited ability to lyse the cells. Finally, the formation of a fully active MAC ring is accomplished by the binding of 12-18 copies of C9.
Fig 1. The formation and electron micrographs of MAC. (Morgan, 2017)
Function of MAC
The foreign microorganism, viruses, fungi, and damaged erythrocytes or nucleated cells can be lysed by MAC. The cytolytic mechanism is considered to be penetration the cell membrane, leading to uncontrolled influx of ions and water down the osmotic gradient, causing the cell to swell and ultimately burst.
The insertion of MAC increases membrane permeability, which is convenient for the ion activity crossing the membrane. Then a lot of intracellular signal pathways are triggered to produce various bioactive substances, including inflammatory mediators (e.g. oxygen free radicals, adipoid) and cytokines. These effects have a positive influence on pathogen clearance and amplification of inflammatory reaction.
The formation of MAC in endothelial cells will promote the expression of p-selectin, which can increase the adhesion of neutrophils, platelets, and monocytes, then accelerate the release of inflammatory mediators.
Recent studies suggest that karyocyte can remove MACs deposited on their cell membranes through vesiculation or endocytosis to protect themselves from complement dissolution.
Fig.2 Schematic diagram of MAC.
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1. Morgan, B. P.; et al. (2017). Molecular cell biology of complement membrane attack. Seminars in cell & developmental biology. 72,124-132.